The present invention relates to an improvement of a method and apparatus for detecting a defective printed matter in a printing press by comparing reference image data with test image data, thereby detecting the defective printed matter.
In recent years, each pixel data (density data) of a printed matter serving as a reference is read and stored as reference image data by using an image processing means having an optical image pickup system, and then each pixel data of a test object is read as test image data by using the same image processing means as described above. The stored reference image data is compared with the test image data to automatically detect a defective printed matter.
The defective printed matter detecting apparatus for a printing press includes an image pickup means such as a CCD camera or a line sensor, a signal processor for processing an electrical signal as an output signal from the image pickup means, and a reference signal generating means for supplying a reference signal for processing to the signal processor. The level of pixel data (pixel position data) of the reference image data is compared with the level of the corresponding pixel data of the test image data to determine whether a difference between the levels of the reference and test image data falls within a predetermined range, thereby determining the defective or nondefective printed matter.
The printed matter serving as a reference for extracting the reference image data is a printed matter which is visually determined as a satisfactory printed matter selected from actual printed matters obtained in test printing at the start of printing.
According to the conventional defective printed matter detecting apparatus, when printing is to be performed in a large quantity and printing actually progresses, the tone of the printed matter is undesirably changed, and this change in tone is often detected to be defective for a further description of the meaning of "tone" as used in the art of printing, please see Ray Blair and Thomas M. Destree, The Lithographer's Manual (Graphic Arts Technical Foundation, Pittsburgh, Pa. 1988) at 11:9-11:19 and 14:22-14:24.
More specifically, in a printing press, printing conditions (e.g., the machine temperature and an ambient temperature) are changed, so that the machine temperature is low at the start of printing and is then increased in continuous printing. When printing progresses, the tone of the printed matter is changed. However, this change in tone does not fall outside a predetermined range and is small, a small change in tone of the printed matter can maintain product values of the printed matters. The printed matters can be circulated as a satisfactory product.
In the conventional defective printed matter detecting apparatus, since the reference image data input at the start of printing is permanent data, a difference between the levels of the reference image data and the test image data is increased while printing progresses. Although the resultant printed matters can be circulated as the satisfactory products, they are undesirably detected as defective printed matters, resulting in inconvenience.